The present invention relates to a process for preparing photothermographic recording materials with an improved stability.
U.S. Pat. No. 3,152,904 discloses an image reproduction sheet which comprises a radiation-sensitive heavy metal salt which can be reduced to free metal by a radiation wave length between an X-ray wave length and a five microns wave length and being distributed substantially uniformly laterally over the sheet, and as the image forming component an oxidation-reduction reaction combination which is substantially latent under ambient conditions and which can be initiated into reaction by the free metal to produce a visible change in colour comprising an organic silver salt containing carbon atoms and different from the heavy metal salt as an oxidizing agent and in addition an organic reducing agent containing carbon atoms, the radiation-sensitive heavy metal salt being present in an amount between about 50 and about 1000 parts per million of the oxidation-reduction reaction combination.
EP-A 386 761 discloses a heat developable color photographic material comprising a support having thereon at least a photosensitive silver halide, a binder, and a dye providing compound capable of releasing or forming a diffusible dye in correspondence or counter correspondence to reduction of the silver halide to silver, said heat developable color photosensitive material further comprising a compound represented by formula (I): C6n(H2O)5n+1 wherein n represents an integer of 1 or more.
There is a constant demand for new ways of stabilizing photothermographic recording materials.
It is an aspect of the instant invention to provide a photothermographic material with improved background density stability.
It is a further aspect of the instant invention to provide a photothermographic material with an improved stability of the photosensitivity.
Further aspects and advantages of the invention will become apparent from the description hereinafter.
Surprisingly, it has been found that by subjecting photothermographic recording materials with at least one layer of the one or more layers of the photo-addressable thermally developable element coated from an aqueous medium to temperatures at or above 35xc2x0 C. in the dark for at least 3 days, there is little or no change in background density Dmin and moreover that the Dmin had been stabilized, as evidenced by the little or no additional change in Dmin upon storage in the dark under conditions such as 7 days at 45xc2x0 C. and 70% relative humidity, which simulate prolonged shelf-life conditions, whether the photothermographic recording material was present as a roll of material or as individual sheets, whereas with conventional photothermographic recording materials with a photo-addressable thermally developable element coated from solvent a loss of image density is obtained under such conditions.
The above-mentioned aspects of the invention are realized by providing a process for preparing a monosheet black and white photothermographic recording material, the photothermographic recording material being exclusive of a dye-providing compound and comprising a support and a photo-addressable thermally developable element, the photo-addressable thermally developable element being thermally developable under substantially water-free conditions and consisting of one or more layers, the layers together comprising photosensitive silver halide, a substantially light-insensitive organic silver salt, a reducing agent therefor in thermal working relationship therewith and a binder, comprising the steps of: (i) coating at least one of the one or more layers of the photo-addressable thermally developable from an aqueous medium; (ii) drying the layer or layers coated in step (i); and (iii) heating the photothermographic recording material at a temperature of at least 35xc2x0 C. in the dark for a period of at least 3 days.
A photothermographic recording material is also provided by the present invention obtainable by the above-mentioned process.
Further preferred embodiments of the present invention are disclosed in the dependent claims.
The term aqueous for the purposes of the present invention means containing at least 60% by volume of water, preferably at least 80% by volume of water, and optionally containing water-miscible organic solvents such as alcohols e.g. methanol, ethanol, 2-propanol, butanol, iso-amyl alcohol, octanol, cetyl alcohol etc.; glycols e.g. ethylene glycol; glycerine; N-methyl pyrrolidone; methoxypropanol; and ketones e.g. 2-propanone and 2-butanone etc.
Substantially light-insensitive means not intentionally light sensitive and resistant to darkening upon exposure.
A monosheet photothermographic recording material is a photothermographic recording material in which all the ingredients for image formation are present in a single sheet and the image formation occurs in that sheet without the assistance of one or more additional sheets.
A black and white and white photothermographic recording material is a photothermographic recording material with which substantially neutral black images are produced.
Exclusive of a dye providing compound means that dyes providing compounds capable of releasing or forming a diffusible or non-diffusible dye are not present
The UAg of an aqueous liquid is defined in this specification as the potential difference between a silver electrode (of 99.99% purity) in the aqueous liquid and a reference electrode consisting of a Ag/AgCl-electrode in 3M KCl solution at room temperature connected with the liquid via a salt bridge consisting of a 10% KNO3 salt solution.
S is defined as that exposure in mJ/m2 at which the photothermographic recording material attained an optical density of 1.0 above Dmin. Thus the lower the value of S, the higher the photosensitivity of the photothermographic recording material.
By the term xe2x80x9cheat solventxe2x80x9d in this specification is meant a non-hydrolyzable organic material which is in solid state in the recording layer at temperatures below 50xc2x0 C. but becomes a plasticizer for the recording layer in the heated region and/or liquid solvent for at least one of the redox-reactants, e.g. the reducing agent for the first silver salt, at a temperature above 60xc2x0 C.
By thermally developable under substantially water-free conditions as used is the present specification, means heating at a temperature of 80xc2x0 to 250xc2x0 C. under conditions in which the reaction system is approximately in equilibrium with water in the air, and water for inducing or promoting the reaction is not particularly or positively supplied from the exterior of the thermographic recording material. Such a condition is described in T. H. James, xe2x80x9cThe Theory of the Photographic Process, Fourth Edition, Macmillan 1977xe2x80x9d, page 374.
A process for preparing a photothermographic recording material, the photothermographic recording material comprising a support and a photo-addressable thermally developable element, the photo-addressable thermally developable element consisting of one or more layers, the layers together comprising photosensitive silver halide, a substantially light-insensitive organic silver salt, a reducing agent therefor in thermal working relationship therewith and a binder, comprising the steps of: (i) coating at least one of the one or more layers of the photo-addressable thermally developable element from an aqueous medium; (ii) drying the layer or layers coated in step (i); and (iii) heating the photothermographic recording material at a temperature of at least 35xc2x0 C. in the dark for a period of at least 3 days. The temperature is preferably at least 40xc2x0 C. and particularly preferably at least 45xc2x0 C. Furthermore, the temperature is preferably less than 50xc2x0 C.
The period is preferably at least 1 week and the heating is preferably carried out at a relative humidity between 10 and 75% and particularly preferably between 15 and 70%. Especially preferred is heating in the dark for 7 days at 45xc2x0 C. and 70% relative humidity.
One of the layers coated in step (i) is preferably coated from an aqueous dispersion comprising a first silver salt, a photosensitive silver halide, a second silver salt and a binder, wherein the aqueous dispersion is substantially free of a water-soluble metal or ammonium salt of an aliphatic carboxylic acid, the first silver salt is a substantially light-insensitive and substantially water-insoluble silver salt of an organic carboxylic acid, and the second silver salt has a solubility in water at 20xc2x0 C. greater than 0.1 g/L. Such treatment surprisingly resulted in photothermographic materials whose Dmin and S-values were both stabilized by the heat treatment according to the present invention, whereas without the addition of the water-soluble silver salt to the aqueous dispersion only Dmin stabilization was observed as a result of the heat treatment according to the present invention.
The silver halide may be any photosensitive silver halide such as silver bromide, silver iodide, silver chloride, silver bromoiodide, silver chlorobromoiodide, silver chlorobromide etc. The silver halide may be in any form which is photosensitive including, but not limited to, cubic, orthorhombic, tabular, tetrahedral, octagonal etc. and may have epitaxial growth of crystals thereon.
The silver halide used in the present invention may be employed without modification. However, it may be chemically sensitized with a chemical sensitizing agent such as a compound containing sulphur, selenium, tellurium etc., or a compound containing gold, platinum, palladium, iron, ruthenium, rhodium or iridium etc., or a combination thereof. The details of these procedures are described in T. H. James, xe2x80x9cThe Theory of the Photographic Processxe2x80x9d, Fourth Edition, Macmillan Publishing Co. Inc., New York (1977), Chapter 5, pages 149 to 169.
The grain size of the silver halide particles can be determined by the Moeller Teller method in the sample containing silver halide particles is sedimented upon a filter paper, which is submerged in electrolyte together with a negative platinum needle-shaped electrode and a reference electrode. The silver halide particles on the filter paper are slowly scanned individually with the needle-shaped electrode, whereupon the silver halide grains are individually electrochemically reduced at the cathode. This electrochemical reduction is accompanied by a current pulse, which is registered as a function of time and integrated to give the charge transfer Q for the electrochemical reduction of the silver halide particle, which is proportional to its volume. From their volume the equivalent circular grain diameter of each grain can be determined and therefrom the average particle size and size distribution.
The photosensitive silver halide used in the present invention may be employed in a range of 0.1 to 100 mol percent; preferably, from 0.2 to 80 mol percent; particularly preferably from 0.3 to 50 mol percent; especially preferably from 0.5 to 35 mol %; and especially from 1 to 12 mol % of substantially light-insensitive organic silver salt.
An aqueous dispersion comprising photosensitive silver halide can be produced using conventional dispersion techniques such as described in Chapter III of xe2x80x9cThe Theory of the Photographic Process Fourth Edition, Ed. T. H. James, Eastman Kodak (1977)xe2x80x9d.
Research Disclosure number 17029, published in June 1978, in section II gives a survey of different methods of preparing organic silver salts. In order to obtain a fine dispersion of an organic silver salt, either the synthesis has to be carried out in an organic solvent medium as disclosed, for example, in U.S. Pat. No. 3,700,458 or in a mixture of water and a substantially water insoluble organic solvent as disclosed, e.g., in U.S. Pat. No. 3,960,908 for silver carboxylates.
An aqueous dispersion comprising substantially light-insensitive silver salts of an organic carboxylic acid, can be prepared by any conventional dispersion technique, for example ball milling, microfluidization, pearl mills etc. together with anionic surfactants, non-ionic surfactants and/or dispersion agents. Alternatively the aqueous dispersion comprising a substantially light-insensitive organic silver salt can be produced directly such as disclosed in EP-A 848 286 and EP-A 754 969.
The water-soluble silver salt in the aqueous dispersion of a preferred embodiment of the present invention has a solubility in water at 20xc2x0 C. of greater than 0.1 g/L, with greater than 1 g/L being preferred.
Suitable water-soluble silver salts include silver nitrate, silver acetate, silver propionate, silver sulfate, silver butyrate, silver isobutyrate, silver benzoate, silver tartrate, silver salicylate, silver malonate, silver succinate and silver lactate, with water-soluble silver salts selected from the group consisting of silver nitrate, silver acetate, silver lactate and silver sulfate being preferred.
It is known in silver halide photography that addition of soluble silver salts to a dispersion of a silver halide produces an increase in UAg (=decrease in pAg=an increase in free silver ion concentration), which can result in partial reduction of the silver salts present, thereby producing metallic silver nuclei. Such metallic silver nuclei give rise to an increased fogging level in silver halide photographic materials. It is possible that an analogous effect is the basis for the sensitivity increase arising from the present invention.
Aqueous dispersion comprising photosensitive silver halide and a substantially light-insensitive organic silver salt can incorporate so-called in-situ silver halide, prepared by conversion of a light-insensitive organic silver salt with a halide ion source such as described in U.S. Pat. No. 3,457,075 and WO 97/48104, so-called ex-situ silver halide in which ex-situ silver halide is either mixed with particles of substantially light-insensitive organic silver salt or is present during the preparation of the particles of substantially light-insensitive organic silver salt as disclosed in U.S. Pat. No. 3,839,049 or by a mixture of in-situ and ex-situ produced silver halide as disclosed in EP-A 922 995.
According to the present invention the photo-addressable thermally developable element is prepared by coating one or more layers with at least one layer being coated from an aqueous medium. The photo-addressable thermally developable element comprises photosensitive silver halide, a substantially light-insensitive organic silver salt, a reducing agent therefor in thermally working relationship therewith, a binder and optionally other ingredients such as spectral sensitizers, supersensitizers, toning agents and stabilizers which assist in the image-forming process or in the stabilization of the resulting image. The element may comprise a layer system with the silver halide in catalytic association with the substantially light-insensitive silver salt of an organic carboxylic acid, spectral sensitizer optionally together with a supersensitizer in intimate sensitizing association with the silver halide particles and the other ingredients active in the thermal development process or pre- or post-development stabilization of the element being in the same layer or in other layers with the proviso that the organic reducing agent and the toning agent, if present, are in thermal working relationship with the substantially light-insensitive silver salt of an organic carboxylic acid i.e. during the thermal development process the reduce agent and the toning agent, if present, are able to diffuse to substantially light-insensitive silver salt of an organic carboxylic acid. The thickness of the thermosensitive element is preferably in the range of 1 to 50 xcexcm.
The photo-addressable thermally developable element may also be coated with a protective layer.
Preferred substantially light-insensitive silver salts of an organic carboxylic acid are those having as their organic group: aryl, aralkyl, alkaryl or alkyl. Aliphatic carboxylic acids known as fatty acids, wherein the aliphatic carbon chain has preferably at least 12 C-atoms, are particularly preferred e.g. silver laurate, silver palmitate, silver stearate, silver hydroxystearate, silver oleate, silver behenate and silver arichidate, which silver salts are also called xe2x80x9csilver soapsxe2x80x9d. Silver salts of modified aliphatic carboxylic acids with thioether group, as described e.g. in GB-P 1,111,492, and silver salts of carboxylic acids described in Research Disclosure 17029, but excluding silver salts of organic carboxylic acids substituted with a heterocyclic thione group as disclosed in Research Disclosure 12542 and U.S. Pat. No. 3,785,830, may likewise be used to produce a thermally developable silver image.
The term substantially light-insensitive silver salt of an organic carboxylic acid also includes mixtures of different silver salts of organic carboxylic acids.
Suitable organic reducing agents for the reduction of light-insensitive organic silver salts are organic compounds containing at least one active hydrogen atom linked to O, N or C, such as is the case with: aromatic di- and tri-hydroxy compounds; aminophenols; METOL(trademark); p-phenylene-diamines; alkoxynaphthols, e.g. 4-methoxy-1-naphthol described in U.S. Pat. No. 3,094,41; pyrazolidin-3-one type reducing agents, e.g. PHENIDONE(trademark); pyrazolin-5-ones; indan-1,3-dione derivatives; hydroxytetrone acids; hydroxytetronimides; hydroxylamine derivatives such as for example described in U.S. Pat. No. 4,082,901; hydrazine derivatives; and reductones e.g. ascorbic acid; see also U.S. Pat. Nos. 3,074,809, 3,080,254, 3,094,417 and 3,887,378. Particularly suitable reducing agents are sterically hindered phenols, bisphenols, sulfonamidophenols and those described in WO97/04357.
Combinations of reducing agents may also be used that on heating become reactive partners in the reduction of the substantially light-insensitive silver salt of an organic carboxylic acid. For example, combinations of sterically hindered phenols with sulfonyl hydrazide reducing agents such as disclosed in U.S. Pat. No. 5,464,738; trityl hydrazides and formyl-phenyl-hydrazides such as disclosed in U.S. Pat. No. 5,496,695; trityl hydrazides and formyl-phenyl-hydrazides with diverse auxiliary reducing agents such as disclosed in U.S. Pat. Nos. 5,545,505, 5,545,507 and 5,558,983; acrylonitrile compounds as disclosed in U.S. Pat. Nos. 5,545,515 and 5,635,339; and 2-substituted malonodialdehyde compounds as disclosed in U.S. Pat. No. 5,654,130
The film-forming binder for use in the photo-addressable thermally developable element of the present invention may be a water-dispersible or a water-soluble binder.
Suitable water-soluble film-forming binders are: polyvinyl alcohol, polyacrylamide, polymethacrylamide, polyacrylic acid, polymethacrylic acid, polyethyleneglycol, polyvinylpyrrolidone, proteinaceous binders such as gelatine, modified gelatines such as phthaloyl gelatine, polysaccharides, such as starch, gum arabic and dextran and water-soluble cellulose derivatives.
Suitable water-dispersible binders are any water-insoluble polymers e.g. water-insoluble cellulose derivatives, polyurethanes, polyesters polycarbonates and polymers derived from xcex1,xcex2-ethylenically unsaturated compounds such as after-chlorinated polyvinyl chloride, partially hydrolyzed polyvinyl acetate, polyvinyl acetals, preferably polyvinyl butyral, and homopolymers and copolymers produced using monomers selected from the group consisting of: vinyl chloride, vinylidene chloride, acrylonitrile, acrylamides, methacrylamides. methacrylates, acrylates, methacrylic acid, acrylic acid, vinyl esters, styrenes, dienes and alkenes; or mixtures thereof.
Preferred water-dispersible binders are water-dispersible film-forming polymers with covalently bonded ionic groups selected from the group consisting of sulfonate, sulfinate, carboxylate, phosphate, quaternary ammonium, tertiary sulfonium and quaternary phosphonium groups. Further preferred water-dispersible binders are water-dispersible film-forming polymers with covalently bonded moieties with one or more acid groups.
It should be noted that there is no clear cut transition between a polymer dispersion and a polymer solution in the case of very small polymer particles resulting in the smallest particles of the polymer being dissolved and those slightly larger being in dispersion.
Water-dispersible binders with crosslinkable groups, e.g. epoxy groups, aceto-acetoxy groups and crosslinkable double bonds are also preferred. Preferred water-dispersible binders for use in the photo-addressable thermally developable element of the present invention are polymer latexes as disclosed in WO 97/04355.
The above mentioned binders or mixtures thereof may be used in conjunction with waxes or xe2x80x9cheat solventsxe2x80x9d also called xe2x80x9cthermal solventsxe2x80x9d or xe2x80x9cthermosolventsxe2x80x9d improving the reaction speed of the redox-reaction at elevated temperature.
The photo-addressable thermally developable element of the photothermographic recording material according to the present invention may contain a spectral sensitizer, optionally together with a supersensitizer, for the silver halide appropriate for the wavelength of the light source which may in the near UV, visible, e.g. 630 nm, 670 nm etc., or IR, parts of spectrum. The silver halide may be spectrally sensitized with various known dyes including cyanine, merocyanine, styryl, hemicyanine, oxonol, hemioxonol and xanthene dyes optionally, particularly in the case of sensitization to infra-red radiation, in the presence of a so-called supersensitizer. Useful cyanine dyes include those having a basic nucleus, such as a thiazoline nucleus, an oxazoline nucleus, a pyrroline nucleus, a pyridine nucleus, an oxazole nucleus, a thiazole nucleus, a selenazole nucleus and an imidazole nucleus. Useful merocyanine dyes which are preferred include those having not only the above described basic nuclei but also acid nuclei, such as a thiohydantoin nucleus, a rhodanine nucleus, an oxazolidinedione nucleus, a thiazolidinedione nucleus, a barbituric acid nucleus, a thiazolinone nucleus, a malononitrile nucleus and a pyrazolone nucleus. In the above described cyanine and merocyanine dyes, those having imino groups or carboxyl groups are particularly effective.
According to the present invention the photo-addressable thermally developable element may further include a supersensitizer. Preferred supersensitizers are selected from the group of compounds consisting of: mercapto-compounds, disulfide-compounds, stilbene compounds, organoborate compounds and styryl compounds.
In order to obtain a neutral black image tone in the higher densities and neutral grey in the lower densities, the photo-addressable thermally developable element according to the present invention may contain one or more toning agents. The toning agents should be in thermal working relationship with the substantially light-insensitive organic silver salt and reducing agent therefor during thermal processing.
In order to obtain further improvement in shelf-life and background density level, stabilizers and antifoggants may be incorporated into the photo-addressable thermally developable element of the present invention.
The support for the photothermographic recording material and aqueous dispersions according to the present invention may be transparent, translucent or opaque and is preferably a thin flexible carrier made e.g. from paper, polyethylene coated paper or transparent resin film, e.g. made of a cellulose ester, e.g. cellulose triacetate, polypropylene, polycarbonate or polyester, e.g. polyethylene terephthalate. The support may be in sheet, ribbon or web form. The support may be subbed with a subbing layer. It may also be made of an opacified resin composition.
The photothermographic recording materials used in the present invention may also contain antihalation or acutance dyes which absorb light which has passed through the photosensitive thermally developable photographic material, thereby preventing its reflection. Such dyes may be incorporated into the photo-addressable thermally developable element or in any other layer of the photothermographic material of the present invention.
In a preferred embodiment the photothermographic recording material of the present invention an antistatic layer is applied to an outermost layer.
Surfactants are surface active agents which are soluble compounds which reduce the interfacial tension between a liquid and a solid. The aqueous dispersions of the present invention may contain anionic, non-ionic or amphoteric surfactants, with anionic and non-ionic surfactants preferred as disclosed in WO 97/04356 herein incorporated by reference. Suitable dispersants are natural polymeric substances, synthetic polymeric substances and finely divided powders, for example finely divided non-metallic inorganic powders such as silica.
The coating of any layer of the aqueous dispersions of the present invention may proceed by any coating technique e.g. such as described in Modern Coating and Drying Technology, edited by Edward D. Cohen and Edgar B. Gutoff, (1992) VCH Publishers Inc., 220 East 23rd Street, Suite 909 New York, N.Y. 10010, USA.
Photothermographic recording materials, according to the present invention, may be exposed with radiation of wavelength between an X-ray wavelength and a 5 microns wavelength with the image either being obtained by pixel-wise exposure with a finely focused light source, such as a CRT light source; a UV, visible or IR wavelength laser, such as a He/Ne-laser or an IR-laser diode, e.g. emitting at 780 nm, 830 nm or 850 nm; or a light emitting diode, for example one emitting at 659 nm; or by direct exposure to the object itself or an image therefrom with appropriate illumination e.g. with UV, visible or IR light.
For the thermal development of image-wise exposed photothermographic recording materials, according to the present invention, any sort of heat source can be used that enables the recording materials to be uniformly heated to the development temperature in a time acceptable for the application concerned e.g. contact heating, radiative heating, microwave heating etc.
Photothermographic recording materials according to the resent invention may be used for both the production of transparencies, for example in the medical diagnostic field in which black-imaged transparencies are widely used in inspection techniques operating with a light box, reflection type prints, for example in the hard copy graphics field and in microfilm applications. For such applications the support will be transparent or opaque, i.e. having a white light reflecting aspect. Should a transparent base be used, the base may be colourless or coloured, e.g. with a blue colour for medical diagnostic applications.
The invention is described hereinafter by way of INVENTION EXAMPLES 1 to 17 and COMPARATIVE EXAMPLES 1 to 6 in which all percentages are percentages by weight unless otherwise specified and the following ingredients were used:
Two silver halide emulsions were used in the INVENTION and COMPARATIVE EXAMPLES of the present invention, with compositions summarized in Table 1:
Type 01 silver halide consisting of 11.44% by weight of 100 mol % silver bromide, with a weight average particle size of 73 nm as measured with the Moeller Teller method (see above for details), and 5.17% by weight of R16875 as dispersing agent in deionized water was prepared at 50.5xc2x0 C. using conventional silver halide preparation techniques such as described, for example, in T. H. James, xe2x80x9cThe Theory of the Photographic Processxe2x80x9d, Fourth Edition, Macmillan Publishing Co. Inc., New York (1977), Chapter 3, pages 88-104.
Type 02 silver halide was produced as for Type 01 except that it contains 3 mol % of silver iodide; it was prepared at 50xc2x0 C. rather than 50.5xc2x0 C.; and a solution of 1 g/L of K2IrCl6. 6H2O was additionally added once 90% of the silver nitrate had been added such than the AgX obtained contained 2.10xe2x88x925 mol Ir4+/mol AgX.